1. Field of the Invention
This invention relates to laser scanning of semiconductor wafers for the purpose of either flowing glass, removing ion implant damage, electrically activating dopants from interstitial to substitutional positions or for increasing grain size of polycrystalline silicon and related activities, and more particularly to a method of using a laser for the purpose of scanning a semiconductor wafer which avoids the previously incurred damage to the wafer edge.
2. Description of the Prior Art
The laser scanning of semiconductor wafers for a variety of purposes, such as reflowing the glass found on the surface of the wafer, removing ion-implantation damage, electrically activating dopants from the interstitial to a substitutional position in the semiconductor material or increasing the grain size of polycrystalline silicon, to name a few, is becoming more widespread in the semiconductor industry. Typically, the wafer to be scanned is placed on a support structure called a "stage" fabricated using a reflective metal such as copper, aluminum, brass or a first metal coated with a reflective material such as gold or silver. The wafer is then mounted on the stage by means of a vacuum drawn through holes formed in the stage beneath the wafer and the combined structure is then subjected to the desired laser beam. A reflective metal is used for the stage so that the laser beam striking the stage is reflected thereby reducing the amount of heating of the stage and preventing the stage from melting. Materials which are not capable of reflecting most of the energy of the laser beam are not suitable for use as the stage. If the reflectivity of the stage is low, then the stage and the wafer on the stage both heat up due to the absorption of heat by the stage thereby changing the annealing parameters of the wafer.
Typically, a laser is scanned across the wafer surface using a raster scan technique. Either the stage containing the wafer or the beam is moved. The wafer is held on the stage by vacuum. Commonly, wafers subjected to laser scan are cracked and otherwise damaged at various points around the edges.